1. Field of the Invention
The present invention relates to a method for micro-fabricating a surface of a base material and a method for manufacturing an optical component whose surface is micro-fabricated such as an optical waveguide, diffraction grating, or polarizer.
2. Related Art of the Invention
Because optical-unit related markets such as those of optical communication, optical disk, display, and optical sensor have been progressed, coexistence of high performance and low cost is requested for an optical component. Particularly, price reduction is requested for a passive optical component that itself does not operate.
Particularly, optical waveguides and diffraction gratings require a very fine and accurate pattern for their surfaces. Dry etching frequently used for a semiconductor process is generally used to form the above pattern. As an example of micro-fabrication by dry etching, a process for manufacturing a single-mode optical waveguide for optical communication is described below by referring to the accompanying drawings.
FIG. 7(a) is a top view of a general quartz-based single-mode optical waveguide and FIG. 7(b) is a sectional view of the waveguide in FIG. 7(a), taken along the line A—A in FIG. 7(a). Because a core 71 has a refractive index higher than that of a clad 72, the light meeting a specific condition is transmitted by being confined in a core pattern. By patterning the core 71 as shown in FIG. 7(a), it is possible to constitute an optical circuit. In a wavelength band of 1.3 to 1.55 μm, the core 71 generally has a square cross section one side of which is approx. 8 μm. A core shape and a core-surface roughness greatly influence a light propagation performance.
FIGS. 8(a) to 8(c) are process charts showing a general manufacture method for a conventional quartz-based optical waveguide (refer to Kawauchi, OPTRONICS No. 8,851, 1988). In the illustrated steps, a core film 81 is formed on a quartz substrate 82 also serving as a lower clad layer in accordance with the flame deposition method (FIG. 8(a)). However, when using a substrate other than a quartz substrate, a lower clad layer is previously formed in accordance with the flame deposition method. Then, a core film is formed into a predetermined pattern through photolithography and dry etching (FIG. 8(b)). An upper clad layer 83 is then formed in accordance with the flame deposition method (FIG. 8(c)). An optical waveguide having a small loss has been manufactured so far in accordance with the above method.
In recent years, however, not only quartz but also resin have been studied as an optical-waveguide material. At present, resin is inferior to quartz in transparency and reliability. Resin is easily molded and superior in transparency at a wavelength of approx. 650 to 850 nm, however. Therefore, it is a very prospective optical-waveguide material. Polymethyl methacrylate (PMMA) superior in transparency is known as a specific resin. Recently, it is also studied to realize low absorption in a wavelength area of 1.3 to 1.55 μm by performing deuteration or fluorination on the basis of PMMA.
To manufacture an optical waveguide by using resin, a method is generally used in which a core layer and a clad layer are formed mainly by spin coating and the core layer is patterned by dry etching. Resin has a high productivity because it has a short film deposition time and a low annealing temperature of 200 to 300° C. compared to quartz.
As described above, dry etching is used to pattern a core of quartz or resin in the case of conventional optical-waveguide manufacturing.
However, dry etching is a complex process and requires a lot of equipment. Therefore, when considering cost, it cannot be said that dry etching is suitable for manufacturing a passive optical component, setting aside a semiconductor device. Therefore, various methods are proposed for manufacturing an optical component. Particularly, pressing-molding and injection-molding methods are prospective.
Proposed examples of press-molding are exclusively directed to glass materials and include a method disclosed in the specification of Japanese Patent Laid-Open No. 8-320420. The method is an optical-waveguide manufacturing method of simultaneously forming groove portions serving as core patterns by pressing a mold 91 on which a predetermined core pattern 92 is formed against a base material 93 also serving as a lower clad at a high temperature as shown in FIG. 9. The method makes it possible to efficiently form an optical waveguide by omitting the photolithography and dry-etching steps which have been used so far.
A method is also proposed in which a diffraction grating is manufactured by transcripting a pattern on a metal reflection film using pressing-molding (the specification of Japanese Patent Laid-Open No. 10-96808).
A micro-pattern transcription method using pressing-molding disclosed in the specification of Japanese Patent Laid-Open No. 8-320420 is briefly described below. First, a base material (workpiece) is heated and softened and brought into contact with a mold. While keeping the above state, the base material is cooled and separated from the mold when the shape of the base material is fixed. In this case, as shown in FIG. 10, a convex portion 103 formed on a mold 101 is provided correspondingly to a required waveguide pattern and pressed against a resin substrate 102, and a reverse pattern is formed on the resin substrate 102.
However, when using the above transcription method, a high pattern accuracy may not be obtained. Deterioration of the pattern accuracy seems to be caused by a difference between thermal expansion coefficients. That is, when bringing the base material softened by heating into contact with the mold and cooling the base material and the mold, a thermal stress is generated due to a difference between thermal expansion coefficients of the base material and mold. As a result, the accuracy of a pattern transcribed to the base material is lowered and the mold is broken if the mold is weak in strength. This problem becomes remarkable when using resin because resin has a thermal expansion coefficient 1 to 2 digits larger than that of quartz used as the material of a mold.
According to specific study by the present inventor, the width of a groove is increased at a micron-order level and the shape of the groove is deformed by transcribing a pattern to a resin substrate by using a mold. It is estimated that this phenomenon results from the fact that the resin substrate contracts toward the center of transcription because the substrate contracts more than the mold. This phenomenon remarkably occurs when a transcription face is a plane. Moreover, when the cross-section of a micro shape is rectangular, a pattern is further deformed.
Thus, resin can be molded at a low temperature and it is advantageous in the manufacturing cost. However, it has a problem that a micro pattern cannot be accurately transcribed when transcribing a pattern by pressing-molding.